The present invention relates to a wafer retainer which is attached to a base plate of a polishing machine so as to hold a workpiece (e.g., a semiconductor wafer) in place while the workpiece is polished, and a method for attaching/detaching the wafer retainer to/from the base plate of the polishing machine.
In recent years, in the semiconductor industry, the integration level of ICs has rapidly increased. It has increased from 4 M to 16 M and is now proceeding to the 64 M level.
In light of the above circumstances, improvement in the surface quality of a wafer (on which ICs are formed) has been demanded. In order to increase the integration level of ICs, it is necessary to reduce the minimum width that is allowed for devices to be mounted on the wafer, as well as improving the chemical and electrical properties of the wafer. The ideal value for the minimum width has shifted from about 0.5 microns to about 0.35 microns.
In order to enable such high-precision processing, more and more stringent demands have been made on the flatness or thickness accuracy of the wafer surface. Specifically, a wafer which has been subjected to a final mirror-surface polishing process is required to have a total thickness variation (TTV) on the order of 1 xcexcm or less over the entire area, and a local thickness variation (LTV) on the order of 0.2 xcexcm or less over an area of about 20 mm2 (i.e., an area generally defining a single IC chip).
In order to satisfy such accuracy requirements, it is necessary to accurately mount a given wafer to be polished on a base plate of a polishing machine so that the wafer is substantially in parallel with respect to the base plate surface. In general, two conventional methods are used for mounting a wafer on a polishing machine base plate.
One method involves applying a fused wax on the surface of a heated base plate so as to allow the wafer to be fixed on the base plate surface via the applied wax.
According to this method, a wafer is first fixed on the base plate surface to carry out a polishing process; after the polishing process is completed, the base plate is heated again to fuse the wax thereon in order to remove the wafer from the base plate surface; and the wafer is washed with an organic solvent to eliminate the wax.
Although this method can provide a satisfactory small level of thickness variation of polished wafers, it requires one heating/cooling step of the base plate before and after polishing. Thus, this method disadvantageously requires a heating step for causing thermal fusing of wax, and hazardous organic solvents for washing off the wax.
Another method for mounting a wafer on a polishing machine base plate involves the use of a wafer retainer, such that the wafer retainer is previously adhered to an upper base plate of a polishing machine, and a wafer is adsorbed to the surface of the wafer retainer via a liquid such as water. This method facilitates a polishing process in terms of attachment/detachment of a wafer to/from a base plate, and therefore provides high efficiency and is suitable for automation. Such a wafer retainer is usually fixed on a polishing machine upper base plate by using a pressure-sensitive adhesive having high tack strength.
In this mounting method, however, the adhesive layer provided on the back face of the wafer retainer substrate is adhered to the base plate surface so firmly that a tack strength as large as about 2 to 3 kg/inch width exists when the wafer retainer thus attached to the base plate must be detached in a usual manner. In other words, a force up to about 57 kg may be required to detach a wafer retainer from a base plate having a diameter of about 485 mm. Thus, it is very troublesome when a used wafer retainer must be exchanged with a new one.
On the other hand, if a wafer retainer fails to be attached in proper alignment with respect to the base plate, it has been difficult to reposition and reattach the wafer retainer.
A wafer retainer according to the present invention includes: a foam layer capable of adsorbing a wafer on a surface thereof in a detachable manner; a first pressure-sensitive adhesive layer formed on a back face of the foam layer; a support formed on a back face of the first pressure-sensitive adhesive layer; a second pressure-sensitive adhesive layer formed on a back face of the support for adhering to a base plate of a polishing machine; and a release sheet releasably attached to the second pressure-sensitive adhesive layer, wherein the second pressure-sensitive adhesive layer includes an adhesive composition, the adhesive composition containing a pressure-sensitive adhesive and a side-chain crystallizable polymer so that the side-chain crystallizable polymer is present in an amount of about 1% to about 30% by weight based on the adhesive composition, and the side-chain crystallizable polymer including as a main component thereof an acrylic acid ester and/or methacrylic acid ester which has a straight-chain alkyl group including 16 or more carbon atoms as a side chain.
In one embodiment of the invention, the adhesive composition contains a tackifier in an amount of about 10% to about 30% by weight; and adhesiveness of the adhesive composition is decreased by more than about 90% when heated above about 50xc2x0 C.
In another embodiment of the invention, the side-chain crystallizable polymer has a molecular weight of about 2,000 to about 15,000.
In still another embodiment of the invention, the side-chain crystallizable polymer has a melting point which occurs within a temperature range narrower than about 15xc2x0 C.
Alternatively, the wafer retainer according to the present invention includes: a wafer retention layer capable of adsorbing a wafer on a surface thereof in a detachable manner; a pressure-sensitive adhesive layer for attaching the wafer retention layer to a base plate of a polishing machine; and a release sheet releasably attached to the pressure-sensitive adhesive layer, wherein the pressure-sensitive adhesive layer includes an adhesive composition, the adhesive composition containing a pressure-sensitive adhesive and a side-chain crystallizable polymer so that the side-chain crystallizable polymer is present in an amount of about 1% to about 30% by weight based on the adhesive composition, and the side-chain crystallizable polymer including as a main component thereof an acrylic acid ester and/or methacrylic acid ester which has a straight-chain alkyl group including 16 or more carbon atoms as a side chain.
In one embodiment of the invention, the adhesive composition contains a tackifier in an amount of about 10% to about 30% by weight; and adhesiveness of the adhesive composition is decreased by more than about 90% when heated above about 50xc2x0 C.
In another embodiment of the invention, the side-chain crystallizable polymer has a molecular weight of about 2,000 to about 15,000.
In still another embodiment of the invention, the side-chain crystallizable polymer has a melting point which occurs within a temperature range narrower than about 15xc2x0 C.
In another aspect of the invention, there is provided a method for attaching/detaching any of the above wafer retainers to/from a base plate of a polishing machine, including the steps of: attaching the wafer retainer to the base plate of the polishing machine by allowing the pressure-sensitive adhesive of the wafer retainer to adhere to the base plate surface maintained at temperature T1; and detaching the wafer retainer from the base plate maintained at temperature T2 which is higher than temperature T1.
The present invention provides the following effects.
According to one embodiment of the present invention, a wafer retainer is provided which includes a pressure-sensitive adhesive layer composed essentially of an adhesive composition containing a pressure-sensitive adhesive and a side-chain crystallizable polymer, where the side-chain crystallizable polymer is present in the amount of about 1% to about 30% by weight based on the adhesive composition. The side-chain crystallizable polymer includes as a main component thereof an acrylic acid ester and/or methacrylic acid ester which has a straight-chain alkyl group including 16 or more carbon atoms as a side chain. As a result, the polymer is capable of shifting between a crystal state and an amorphous state in a reversible manner, responsive to a relatively small temperature change from an arbitrarily selected ordinary temperature value; consequently, the polymer exhibits drastic a change in its tackiness to the base plate surface.
Thus, when the wafer retainer is heated to a predetermined elevated temperature (e.g., about 50xc2x0 C. to about 100xc2x0 C.), the adhesiveness of the adhesive layer rapidly decreases. Therefore, at the time of exchanging wafer retainers, the base plate and/or the wafer retainer may be heated so as to greatly decrease the adhesiveness of the adhesive layer with respect to the base plate surface, whereby the wafer retainer can be easily peeled off the base plate surface.
In accordance with an embodiment of the invention where the adhesive composition contains a tackifier in the amount of about 10% to about 30% by weight and where the adhesiveness of the adhesive composition is decreased by more than about 90% when heated above about 50xc2x0 C., a predetermined level of adhesiveness with respect to the base plate surface can be retained at ordinary temperatures, whereas a rapid decrease in adhesion strength can be caused by heating. Thus, a tackifier present in the above-mentioned amount will not substantially influence the temperature sensitivity of the polymer. As a result, the adhesive composition according to the present invention exhibits a sufficient adhesion strength at ordinary temperatures, while maintaining a good balance between the adhesion strength at ordinary temperatures and the peeling force required at elevated temperatures.
Thus, the invention described herein makes possible the advantages of (1) providing a wafer retainer which adheres strongly, stably, and precisely to a base plate of a polishing machine while a wafer or the like is polished with the wafer retainer, and yet allows for easy detachment from the base plate; (2) providing a method for attaching/detaching such a wafer retainer to/from a base plate of a polishing machine; (3) providing a wafer retainer which can be easily repositioned and reattached for proper. alignment with respect to a base plate of a polishing machine; and (4) providing a method for attaching/detaching such a wafer retainer to/from a base plate of a polishing machine.